Mobile telecommunication networks comprise a plurality of network cells each having at least one base station used to receive and transmit signals from user equipments (UEs), e.g. mobile phones or Personal Digital Assistants. A plurality of different network environments or network systems such as for instance GERAN, UTRAN, LTE, E-UTRAN, WCDMA or WLAN is known. For ensuring a good network performance and in particular an effective data transmission it has to be ensured that all data, data signals or data packets are received at the intended recipient. Depending on the direction of a data transfer the recipient may be a UE or a base station (BS). In case of a relay enhanced network the recipient may also be a relay station or a relay node.
One problem limiting the performance of the data transmission known in the prior art is inter cell and intra cell interference. In order to reduce the inter cell interference so called cooperative antenna (COOPA) networks are proposed. In a COOPA network a coordinated processing is carried out for at least two BSs and at least one UE is simultaneously provided with data on the same radio transmission resource (Physical Resource Block, PRB) from the at least two coordinated BSs. Thereby, the Signal to Interference and Noise Ratio (SINR) for the UE can be increased significantly. From theory significant performance gains with respect to capacity and coverage are known for full cooperating cellular radio networks compared to conventional radio networks. It is known from theory that these large gains cannot be achieved with other technologies. Therefore, COOPA networks provide an upper bound for interference limited cellular radio networks. As a consequence, it is very likely that at least some form of cooperation will have to be implemented in future radio telecommunication networks.
In the meantime different types of COOPA networks have been proposed. However, within this application a basic COOPA cell of a cooperation area (CA), which comprises two cooperating BSs and two UEs will be discussed.
FIG. 2 schematically illustrates a basic solution for cooperative joint transmission, which is helpful for understanding of the invention described in this application. In particular, a central unit (CU) may be foreseen for COOPA networks to perform a joint precoding and—as the name suggests—may be placed at a central point of the CA at one of the cooperating BSs. The other cooperating BS may be connected to this CU by fast and low delay optical fiber connections.
The CU may perform in downlink (DL) a common signal precoding like joint transmission, which is basically a matrix multiplication of all data signals for all cooperating UEs with a precoding matrix W. In case of zero forcing (ZF), wherein the precoding is carried out in such a manner that there is no interference, the precoding matrix W is the pseudo inverse H+ of the overall channel matrix H. The simplest form of a CA for a codebook based precoding is illustrated in FIG. 2. In this case, the precoding matrix W is selected from a codebook based on the estimated radio channels between all involved UEs and BSs. This can be done either by using the uplink (UL)-downlink (DL) reciprocity with respect to the corresponding electromagnetic pulse propagation (for TDD systems) or by an explicit signaling by the UE1 and the UE2. Taking into account the described channel reciprocity may provide the advantage that the feedback overhead can be reduced significantly. Therefore, this technique is an interesting candidate for performing cooperation.
In particular, FIG. 2 shows data packets d1 and d2 to be transmitted to UE1 and UE2. For the common signal processing the data packets are encoded by using the matrix W to form the data signals tx (tx1, tx2) to be transmitted to the BS1 serving UE1 and to the BS2 serving UE2, where signals r1 and r2 are received, respectively. The signals r1 and r2 correspond to the multiplication of the channel matrix H, the pseudo inverse H+ or W and the data d to be transmitted offset by an offset vector n.
Compared to Frequency Division Multiplex (FDD) systems TDD systems are prone to higher interference as BSs receive interference from other BSs or UEs that transmit during the same time. Additionally, interference can be caused by large propagation delays and by signals arriving at times after a main signal component has arrived. This holds in particular if moderate guard intervals or short cyclic prefixes in connection with large coverage areas are used. For this reason, in order to reduce such interference in such systems strong beamforming will be combined with synchronized transmissions from all BSs.
In order to reduce interference within TDD systems, it is currently planned to use identical switching points for all BSs. In this context a switching point is defined by the point in time, when a BS changes from UL reception to DL transmission or vice versa. However, identical switching points for all BSs have the disadvantage of reducing the TDD adaptability to DL-UL traffic asymmetries (i.e. DL and UL transmission are of different length.
There may be a need for adapting COOPA techniques to allow for variable switching points in TDD systems, while simultaneously exploiting well known COOPA gains from known FDD systems.